A1StereoControl

With my A1StereoControl plugin you will be able to expand or limit the STEREO WIDTH of your tracks using only one single knob. This powerful technique can be used on single tracks or groups tracks while mixing or even on a master bus in final mastering situations.
As many experienced mixing & mastering engeneers worldwide are complaining, stereo-expansion can have unwanted side-effects. Especially the bass frequencies should normally be simple mono.
As you may guess, my plugin has also a solution for this problem. The integrated "Safe Bass" algorithm centers all bass frequencies below a adjustable value right back into the middle of the stereo field. This gives your tracks the solidity and definition you want while preserving maximum transpancy and sharpness. Similar techniques were used by professional studio producers over ages
with either expansive hardware devices like elliptical equalizers or even advanced m/s-processors. With my plugin you will be able to manage this complicated task with a minimum effort.

http://www.alexhilton.net/A1AUDIO/index.php/a1stereocontrol

RealSpread

Truality Audio has released RealSpread, a free VST plugin that widens a sound source.

RealSpread "Stereoizes" a mono audio signal or widens a stereo one, without practically compromising mono compatibility. The spartan GUI conforms to RealSpread's philosophy of simplicity in plugin design.

RealSpread is more effective on single sources, for example a single (mono) Drums Overhead or Room microphone. Of course, you can experiment with stereo sources.

https://trualityaudio.weebly.com/free-vst-plugins.html

Polyverse Wider

Wider takes the massively popular stereo section from its sibling, Manipulator, and creates the illusion of an expanded stereo image of a mono signal to an awe-inspiring amount of width. However, Wider is a unique stereo plugin in the sense that it is completely “mono-compatible”, meaning that any signal that has been extended will always remain in phase with itself, even if summed to mono.

Thanks to Wider’s one-of-a-kind all pass and comb filtering algorithm, a natural sounding stereo image is created from a mono channel, without compromising the phase between the sides! Instead, Wider cancels itself out when summed to mono, so the original signal is left intact.

Wider can be used to increase the stereo image of any mono signal up to 200% of full stereo for an extra wide, exaggerated effect.

https://polyversemusic.com/products/wider/

Ozone Imager

Ozone Imager is a FREE stereo imaging plug-in for your DAW or NLE. Born from Ozone’s legendary Imaging technology, Ozone Imager can narrow or widen your stereo image with simple controls, stunning visuals, and incredible sound.
Download it today and turn narrow, dull audio into a full, impactful track.

https://www.izotope.com/en/products/master-and-deliver/ozone-imager.html

Multiband Stereo Image Modifier Lite

Using Multiband Stereo Image Modifier Lite you can freely change the sense of space, in 3 different frequency-ranges. Mid/Side mode optimalizes the audio sign. You can change the volume or stereo image with the Mid/Side volume functions.
Graphic frequency range handler interface: It can manage 3 different frequency ranges. The overlay of the different ranges can be changed with the vertical white lines. The ranges are colour-coded. When used, the vectorscope shows the stereo images of the ranges, and therefore you can see on the graphic display if the sense of space is correct.
Side volume: Adds to the sense of space. In middle-state there is no change. Pushing the slider up the sign changes to mono, pushing it down adds to the sense of space. Deafult value is 128.
Mid volume: Changes the volume of the mid-channel Useful function, because it lowers the middle sounds’volume – like the kickdrum or the vocal zone. Default volume is 256.
Volume: All 3 frequency-ranges’ volume can be changed separately.
Modifier on/off: In off mode the switch turns off all the effects in the program.
Band buttons: These buttons are used to turn on and off the different frequency-ranges. When turned off, it turns off the whole stream of signs from the ranges – meaning that both effects and the original sign will be turned off. In this state the frequency-ranges’ interface can’t be seen, therefore it’s easy to tell which range is on or off.

http://www.bitsonic.eu/stereoimagerlite.html

MaxSynths has released Sturiophonia, a free stereo image widener and sound placement VST plugin for Windows.

The aim of the plugin is to increase the stereo image of mono tracks and help to place the sound in the right place by the use of dedicated internal filters and pan controls. The idea behind it is very simple yet effective and adds to the sound a pleasant stereo effect.

The best way to use this plugin is with mono tracks. It's possible to switch the main input from mono to stereo (for example if you want to use it on the main mix).

http://www.maxsynths.com/vst_free_sturiophonia.html